1. Field of the Invention
This invention relates to a process for preparing selective permeable membrane useful in reverse osmosis and ultrafiltration that has the ability to selectively separate the solvent from liquid mixtures such as solutions, emulsions and suspensions.
2. Description of the Prior Art
Typical examples of a selective permeable membranes which are only permeable to a specific component of a solution and emulsion are a reverse osmosis membranes and an ultrafiltration semipermeable membranes. The reverse osmosis membranes can separate the solvent from a solution containing matter of a relatively small particle size or substances of a low molecular size such as sodium chloride and is used in treating waste water from plants, purifying sewage and desalination of sea water and brine.
The ultrafiltration membrane, on the other hand, is capable of separating the solvent or dispersion medium from a solution or emulsion containing substances of a relatively large particle or molecuar size such as colloids, proteins or microorganisms, or a polymer and is used in purification and concentration steps involved in the manufacture of foods and medicines as well as in the brewing and fermentation industries.
Heretofore, selective permeable membranes of this kind have been made from cellulose acetates, polyamides, polysulfones, etc. However, selective permeable membranes made from cellulose acetates and polyamides are not only low in heat resistance but they also have poor resistance to chemicals such as alkali and strong acids, which frequently make up the liquid mixture treated by the membrane.
Permeable membranes made from polysulfones have more heat and chemical resistance than those made from cellulose acetates and polyamides but they have poor resistance to organic solvents.
An attempt has recently been made to make a selective permeable membrane using an aromatic polyimide made up of an aromatic tetracarboxylic acid and a diamine, but the process requires converting the amide acid moiety in the molecular skeleton of the resulting selective permeable membrane to an imide ring at high temperature and this additional step gives the product low permeability.